1. Field of the Invention
The invention relates to the determination of the fracture strength of thin, flat samples of brittle-fracture material, in particular thin glass sheets, under a tensile stress σ.
2. Description of Related Art
It has been found that two mutually independent kinds of fractures can occur when such samples are placed under tensile loads: fractures that have their origin inside the surface area of the sample and those that grow starting from the margin of the sample. The present invention relates to the latter kind of fractures, that is, to those that have their origin at the margin of the sample.
Both for characterization and optimization of the properties of the edge and for ensuring a guaranteed fracture strength, it is advantageous to examine whether the margin of a sample withstands a certain tensile stress σ.
Moreover, it is advantageous to determine the stress σb of such a sample, in which the sample fractures starting from the margin (tensile stress at break).
A two-point bending method is known for the determination of the tensile stress at break of thin glass samples from, for example, S. T. Gulati: “Two Point Bending of Thin Glass Substrate,” SID Symposium, Techn. Papers Vol. 42, pp. 652-654 (2011). In this case, a thin glass sample is clamped between two support plates and bent by bringing these plates together (compare FIG. 1).
This method has a number of drawbacks. In this kind of bending, an inhomogeneous state of stress is created along the sample, with the highest stress being imposed along the margin in the middle of the sample and the stress declining with increasing distance from the middle. Therefore, this method is not adequate for characterizing extended sections of sample margins under a given stress. The generalization of the measured values of local tensile stress at break to larger sections with this method has been demonstrated to be reliable only to a limited extent. Moreover, samples with inhomogeneous thickness cause problems in this method, which can be solved only with difficulty. Furthermore, the necessity of clamping places requirements on the geometry of the samples and, in many cases, makes necessary a tedious preparation of the samples. In addition, the cost in terms of instruments and personnel required to carry out this method cannot be underestimated.